3 Answers
3

The sender can put anything he wants in the To and From headers. The SMTP server will deliver to the address defined with RCPT TO command. If your spam filter or mail server doesn't block it, it will get delivered.

It is probably possible to write an email like this in code although I have never tried it. You can also talk to email servers directly and create email on them using telnet. There are many examples on the web, one is here.